The present invention relates to an optical waveguide array having the structure that a plurality of domains where characteristic absorption in a wavelength region longer than 360 nm decreases together with change of a refractive index are continuously formed in inner parts of material, and a method of manufacturing such the optical waveguide array.
An optical waveguide array having optical fibers installed in a substrate is used as a means for digital and/or image data in an optical communication system. A conventional optical fiber has the structure that a core of a higher refractive index is surrounded with a cladding layer. Due to such the structure, incident light which is emitted to the optical fiber with an angle less than a numerical aperture (NA) repeats total reflection at an interface between the core and the cladding layer, to transmit image data toward an outlet of the optical fiber.
However, light which is emitted to the optical waveguide array with an incidence angle greater than a value corresponding to the numerical aperture (NA) does not perform total reflection at the interface between the core and the cladding layer, but travels through the cladding layer to an adjacent optical fiber. Light emitted to the cladding layer also travels through the cladding layer and the core, and reaches the opposite side. Such the unfavorable travelling causes occurrence of so-called xe2x80x9ccross-talkxe2x80x9d that the light travels in the part where travelling shall be originally forbidden, resulting in frequent occurrence of errors in transmission of digital data, and decrease of contrast as well as degrading of image in case of transmission of image data.
Cross-talk is suppressed by provision of a light absorber between optical fibers of an optical waveguide array to absorb leaked light, as disclosed in JP 1-180180A and JP 3-38963A. In such an optical waveguide array (as shown in FIGS. 1A, 1B and 1C, herein collectively referred to as FIG. 1), each core 1a is surrounded with a cladding layer 1b and a light-absorbing layer 1c, a plurality of the optical fiber 1 are bound together as bundles 2, and each bundle 2 is individually sandwiched between substrates 3 such as glass. Since leaked light is separated by the light-absorbing layer 1c, an image is not degraded of contrast during travelling, so that an image sensor capable of reading image data with high resolution is offered.
However, there are restrictions on material of the light-absorbing layer 1c, since the optical fiber 1 covered with the light-absorbing layer 1c shall be good of adhesiveness to glass. In addition, a very complicated process is necessitated due to formation of the light-absorbing layer 1c as well as adhesion of bundled optical fibers 1 to the substrates 3.
European Patent No. 0797112A discloses production of an optical waveguide by irradiation of a glass sample with a laser beam condensed at a focal point in an inner part of the glass sample to partially increase a refractive index at the focal point. In this method, a quartz or fluoride glass is irradiated with a condensed laser beam to form an optical waveguide. Production of an optical waveguide array is anticipated in course of developing such the method to enable formation of optical waveguides in an arrayed state. However, condensed irradiation with the laser beam merely induces change of an refractive index, but cross-talk is still unresolved. Consequently, image data are transmitted in a degraded state with poor contrast.
The present invention aims at elimination of above-mentioned problems. An object of the present invention is to provide a new optical waveguide array having the inner structure that a plurality of domains where a change of a refractive index as well as decrease of characteristic absorption in a longer wavelength region occur are continuously formed by irradiating a glass, which contains an absorbing material with characteristic absorption in the longer wavelength, with a pulsed laser beam condensed at a focal point preset in inner parts of the glass.
An optical waveguide array according to the present invention comprises a glass matrix containing an absorbing material with characteristic absorption in a wavelength region longer than 360 nm, and a plurality of domains, where change of a refractive index as well as decrease of characteristic absorption in a wavelength longer than 360 nm occur due to transition of the absorbing material caused by irradiation with a pulsed laser beam condensed at a focal point preset in inner parts of a glass, are continuously formed in the matrix. The absorbing material may be one or more of metal microparticles, semiconductor microparticles, transition metal ions, rare earth ions and anions.
The optical waveguide array is fabricated as follows: A pulsed laser beam with an energy capable of inducing change of a refractive index as well as decrease of characteristic absorption in a wavelength region longer than 360 nm is emitted to a glass containing an absorbing material with characteristic absorption in the wavelength region longer than 360 nm, in the manner such that a focal point of the pulsed laser beam is adjusted to an inner part of the glass. Such irradiation is continued while relatively shifting the focal point in the glass, so as to form a continuous domain where change of the refractive index as well as decrease of characteristic absorption in a wavelength region longer than 360 nm occur in the inner part of the glass. Such the domain serves as an optical waveguide. After the focal point is relocated, the same irradiation is repeated to form a plurality of optical waveguides.
When a glass containing an absorbing material with characteristic absorption in a longer wavelength region is irradiated with a pulsed laser beam condensed at a focal point preset in an inner part of the glass, change of the refractive index as well as transition of the absorbing material occur at the focal point. Such an absorbing material as metal microparticle, semiconductor microparticle, transition metal ion, rare earth ion or anion has characteristic absorption in a wavelength region longer than 360 nm. Condensed irradiation with the pulsed laser beam also changes a number of metal microparticle or semiconductor microparticle, and a size or transformation of the microparticle. Such the condensed irradiation also changes valence, coordination and integration of transition metal ion, rare earth ion or anion.
For instance, when a glass dispersing metal microparticles or semiconductor microparticles therein is irradiated with a condensed pulsed laser beam, the microparticles are decreased in number, reduced in size, or dissolved or ionized in a glass matrix.
Absence of the microparticles due to such dissolution or ionization causes decrease of an absorption coefficient to the same value as that of a glass free from dispersion of the microparticles, compared with a level before irradiation. Change of the microparticles in size causes change of absorption wavelength, i.e. decrease of an absorption coefficient compared with a level before irradiation.
A part subjected to condensed irradiation increases its refractive index compared with the other part which is not subjected to condensed irradiation, due to structural re-arrangement caused by the condensed irradiation, so that structure of an optical waveguide is formed in the glass. When a laser beam for transmission of image data with wavelength adjusted to a wavelength region of characteristic absorption is emitted to the processed glass, the laser beam travels along the optical waveguide at a high efficiency, since an absorption coefficient is decreased at the focal point while the other part keeps its original absorption coefficient before the condensed irradiation. In addition, light leaked out of the waveguide (the irradiated part) is trapped in the non-irradiated part, so as to inhibit occurrence of errors in data transmission. Consequently, image data can be read out with high resolution without degrading of contrast.